Modulation system



March 10, 1936. M. G. CROSBY MODULATION SYSTEM Original Filed Sept. 19, 1951 3 Sheets-Sheet l POWER HER! AND

FREQUENCY MULTIPUER AMPLI CARNER SOURCE POWER Ana FREQUKNCY MULTIPLE AMPLIFIERS LIMIT! R in N R A C POWER MUL'HPHER a R U 0 5 AMPLmERs manna AND FREQUENCY CARNER SOURCE RADIO FREQUENCY CHOKE COIL lNVENTOR MURRAY 6. I say BY ATTORNEY March 10, 1936. M. G. CROSBY MODULATION SYSTEM Original Filed Sept. 19, 1931 3 Sheets-Sheet 2 5 i 1% power.

1 1 marred 1112 cmmaa B 1 FREQUENCY I Mun-mum i mono FREQUENCY cnoxs colLs v 1.9 1 AMPHFIERS uurms: mo FREQUENCY Ll; MULTIPLIERS MODULATING CHOKE cou INVENTOR +6 MURRAY 6- c SBY I, MvI/V ATTORNEY March 10, 1936.

M. G. CROSBY MODULATION SYSTEM 3 Sheets-Sheet 5 Original Filed Sept. 19, 1931 To MODULATlNG CHOKE DULATING CHOKE I T0 No RADIO FREQUENCY CHOKE COlL INVENTOR MURRAY 6 BY CRBY I ATTORNEY Patented Mar. 10, 1936 UNITED STATES 2,033,231 MODULATION srsrnitt Murray G. Crosby, Rlverhead, N. assignor to Radio Corporation of America, a corporation of Delaware Application September 19, 1981, Serial No. 563,725

Renewed June 20, 1935 10 Claims. (Cl. 179-171) This invention relates to signal transmission systems and more particularly to systems directed to, the production and transmission of modu 'lated' electrical waves.

cordance with the message waves to be transmitted. This modulation may refer to a variation in phase, frequency or amplitude of a high frequency carrier wave where the variation occurs in accordance with signal impulses such as may be caused by sound or electrical waves of a relatively low frequency.

Heretofore, phase modulation has been accomplished by somewhat complicated circuit arrangements which are inserted in the transmitting apparatus. These arrangements, it has been found, are rather expensive and complex, and require careful maintenance.

Accordingly, a primary object of this invention is to secure an improvement in the method of and apparatus for modulating electrical waves. A

more specific object is to provide a simple and,

efiicient phase modulation arrangement which may be connected to any existing transmitter with very few changes.

According to one particular embodiment of this invention, these objects are attained by utilizing capacitive and inductive couplings between the modulating device and the output of the carrier current amplifying apparatus. According to another embo'diment, the phase of the carrier wave is altered by changing the characteristics of the transmission line over which the wave is propagated.

Various features of this invention reside in the different circuit arrangements employed for coupling the modulating device to the transmitter apparatus and will appear in the subsequent detailed disclosure.

Referring to the drawings:

Fig. l is a circuit diagram of a transmitting system adapted for phase modulation wherein a signal modulating tube is coupled to the carrier frequency amplifier by means of a capacitive coupling;

Fig. 2 is an arrangement similar to Fig. 1 except that inductive coupling is employed instead of capacitive coupling;

Figs. 3 and 4 are further modifications of this invention and disclose differential modulator circuits;

Fig. 5 is a schematic diagram of a complete transmitting system employing an artificial line. Phase modulation is attained in this circuit by varying the capacity per unit length of the artificial line;

Figs. 6, 7 and 8 are views illustrating various specific circuit schemes for connecting the artificial line of Fig. 5 to the other portions 01' the system.

. In Fig. 1 is shown a complete radio transmitting system ,adapted for the transmission of high frequency electrical waves. Numeral l0 represents a suitable antenna circuit connected to the system by suitable power amplifiers and frequency multipliers. These amplifiers and multipliers are not/shown o'r described herein since they are well lr nown in the art and form no part of this invention. The waves to be transmitted by antenna circuit in are generated by carrier frequency source I which is connected by means of a transformer 2 to an amplifier tube 3. The tuned circuit in the output of the amplifier tube 3 comprises a condenser i2 and inductance l3 con-- nected in shunt to said condenser. This output circuit is associated with the power amplifiers and frequency multipliers which are indicated in the rectangular box. Directly connected to the anode of the amplifier tube 3 is a condenser I 4, one side of which is connected directly to the anode of modulating tube ll Although the amplifier tube 3 shown in the drawings is a screen grid tube and modulating tube H is a three element tube, it is to be understood that any type of tube, whether a triode, pentode, or screen grid, may be employed. A radio frequency choke coil l5, which is of high impedance to the carrier frequency, connects the plate voltage with the amide of tube ll. A transmitter l6 which may comprise any suitable modulation input device is employed to produce the signal waves which are fed through transformer I! to the modulator tube ll.

Condenser I4, it is to be noted, is connected in series with the plate impedance of modulator tube It and effectively connected in parallel with condenser l2. Consequently, any variation of the plate impedance of tube I i such as may be caused by message waves impinging on the diaphragm of transmitter IE will cause a variation of the effective capacity of condenser M. This variation of the effective capacity of M will vary the resultant capacity of the tuned circuit I 2, iii in such manner that the output of the amplifier is phase modulated in accordance with the signal wave; Any amplitude modulation caused in this circuit which might be undesirable may be removed in the. system by employing limiters or over-loaded amplifiers whose output is limited to a definite value regardless of how much the input is increased. An ordinary vacuum tube amplifier willserve this purpose if it is over-loaded in such manner that its output does not increase in proportion to its input. Such limiting devices are well known in the art and therefore need not be illustrated here. If desired, frequency multi- -modulatcr' circuit through a capacity.

pliers may be used to multiply the phase displacement of the'modulated wave.

Fig. 2 illustrates a circuit arrangement similar in many respects to that of Fig. 1 except that the effective inductance of the tuned circuit l2, 13, instead of the capacity, is modulated. This is accomplished by means of inductance coil l8 in the output of modulator tube H which is coupled to the tuned circuit inductance coil l3. In this particular arrangement the radio frequency choke coil I is omitted from the output circuit of the modulator tube. Any variation of the plate impedance o'f modulating tube l I will vary the phase of the amplifier output in accordance with the signal wave in the same manner as mentioned above in connection with the description of the operation of the circuit of Fig. 1

Inasmuch as modulating the efiective inductance of a tuned circuit modulates the phase of the electrical waves in a direction opposite to that of modulating the capacity, difierential modulator circuits such as are illustrated in Figs. 3 and 4, may be employed to cancel non-linearities in either type of modulator. In Figs. 3 and 4 the plate circuit of one modulator tube is coupled to the output amplifier circuit through an inductance coil and the plate circuit of the other modulator tube is coupled to the output of the same The modulating voltage is fed' to the two modulator tubes 23 and 24 which are in push-pull relationship 180 degrees out of phase so that while tube 24 is modulating to increase the efiective capacity of the tuned circuit I2, 63, tube 23 is modulating to increase the efiective inductance of the same tuned circuit. At the instant that the plate impedance of tube 23 is low, that of tube 24 is high so that tube 24 tends to release the shunting eifect of condenser l4, thereby decreasing the total effective capacity, and tube 23 tends to increase the shunting effect of coil l8, thereby decreasing the total effective inductance. In this manner the phase of the output of the amplifiers is modulated by modulating the effective capacity and inductance of the tuned circuit l2, l3 simultaneously. The operating characteristics of the differential modulators in Figs. 3 and 4 may be controlled in any desired manner by adjusting the values of condenser I4 and coil l8 and the inductive coupling of coil E8 to coil l3.

Although the differential modulator circuits disclosed in Figs. 3 and 4 are shown coupled to the same amplifier, it is to be understood that, if desired; one of the modulators may be coupled through an inductance to the tuned circuit of the first amplifier tube and the other modulator coupled through a capacity to the tuned circuit of the second amplifier tube. The two modulations would then add together to produce the sum of the separate modulations. In the same manner any number of these phase modulators may be applied to various stages of amplification in the same transmitter to increase the depth of modulation or to improve its characteristics.

Fig. 5 illustrates a transmitting system adapted for phase modulation wherein the distributed constants of an artificial line are varied in accordance with the signal wave. An artificial line 20 shown in'box A is connected to the difierent parts of the system over leads a, b, c, d and e. In this arrangement glow discharge tubes l9, l9 are utilized as variable condensers and form part of the distributed capacity of the line. These tubes may be neon, tubes, cathode ray tubes or any suitable type of tube wherein an electron stream may be varied to vary the capacity between the electrodes of the tubes and the metallic plate intermediate said electrodes. With no current through the tube the capacity of the tube will only consist of the capacity between the electrodes and the metallic plate. The application of a potential between the electrodes of the tube will cause a current to flow between the electrodes and a glow to strike which will, in turn, act as a conducting material between the electrodes. A variation of the potential across the tube will vary the glow within the tube with a consequent variation in capacity of the tube. Thus, the variation in impedance in the plate circuit of modulator tube II will affect the glow tubes and vary their capacity in accordance with the signal wave in an obvious manner. A plurality of radio frequency choke coils 2!, 2| of high impedance to the carrier frequency are serially connected in the glow tube circuits. Phase modulation in this arrangement is obtained by continually changing the velocity of the carrier wave transmitted over the artificial line by changing the constants of the line.

One advantage of this arrangement is that the length of the line may be increased to increase the amount of phase deviation with signal, thus obtaining a decrease in the percentage of variation of the variable distributed constants and consequently obtaining a more linear modulation.

Another advantage of this particular arrangement is that an increase in phase deviation may be obtained by lengthening the artificial line. In such case, frequency multiplication will not be required.

Figs. 6, 7 and 8 illustrate difierent modulator units which may be employed to replace the apparatus in box A of Fig. 5 in the complete transmitting system.

Referring to Fig. 6, there is shown a glow tube 2| in the form of a variable condenser within which an artificial line is helically wound. If desired this line may be wound around the outside of the tube in very close proximity to the tube. The capacity of the line to ground is varied by changing the glow tube current, thus causing phase modulation of the carrier current impressed on the artificial line.

In Fig. 7 the efiective inductance per unit length of the artificial line is modulated by modulating the plate impedance shunted across the inductances 22, 22 which are coupled to the artificial line at various points. 'In this manner the velocity of the carrier wave propagated along the artificial line is varied in accordance with the variation of the signal wave with a consequent modulation in phase of the carrier.

In Fig. 8 is an arrangement whereby the plate impedance of the modulator tube is serially connected with the capacity of the line to ground. A variation of this plate impedance efiectively varies the capacity of the line to ground.

It should be understood, of course, that this invention is not limited to the specific embodiments disclosed herein, but that various different organizations may be employed which embody the principles of this invention without departing from the spirit and scope thereof.

What is claimed is:

1. A phase modulation signalling system com- T quency oscillations, a tuned circuit connected be tween the anode and cathode of said tube, and means for varying the tune of said circuit at signal frequency to thereby vary the phase of the oscillations relayed in said tube and circuit comprising a thermionic tube having its input electrodes energized at signal frequency, and its anode electrode coupled by way of a reactance to said tuned circuit.

2. A device for producing and relaying carrier frequency oscillations and for modulating the phase of the oscillations relayed at signal frequency comprising, a source of carrier frequency oscillations, a thermionic tube having its control grid and cathode electrodes coupled to said source to be energized at the frequency of said carrier frequency oscillations, a tuned circuit comprising an inductance and condenser in parallel connected between the anode and cathode of said tube, and means for varying the tune of said tuned circuit at signal frequency to thereby vary the phase of the oscillations relayed in said tube and circuit comprising a thermionic tube having its input electrodes energized at signal frequency, its cathode connected to the cathode of said first named tube and its anode electrode connected by way of a reactance to a point on said circuit including said inductance and capacity in parallel.

3. A device for producing and relaying carrier frequency oscillations and for modulating the phase of the oscillations relayed at signal frequency comprising a source of carrier frequency oscillations, a thermionic tube having its input electrodes coupled to said source to be energized at the frequency of said carrier frequency oscillations, a circuit including an inductance and variable condenser in parallel connected between the anode and cathode of said tube, a connection between the cathode of said tube and ground, a source of modulating potentials, a modulating tube having its control grid and cathode coupled to said source of modulating potentials, a connection between the cathode of said modulating tube and ground, and a capacitive reactance coupling the anode of said last named tube to the inductance of said parallel circuit to vary the tune thereof in accordance with said modulating potentials.

4. A device for producing oscillations and modulating the phase of said oscillations at signal frequency comprising, a source of oscillations,

a thermionic tube having input electrodes connected to said source of oscillations to be energized at the frequency of said oscillations, a tuned circuit including an inductance and variable conde'nser in parallel connected between the anode and cathode of said tube, a modulating tube having its control grid and cathode coupled to a source of modulating potentials, and an inductive reactance coupling the anode of said last named tube to the inductance of said parallel circuit to vary the tune thereof in accordance with said modulating potentials.

5. A device for relaying carrier frequency oscillations and for modulating the phase of the oscillations relayed at signal frequency comprising, a thermionic tube having its input electrodes energized at the frequency of said carrier frequency oscillations, a tuned circuit including a reactance connected between the anode and cathode of said tube, and means for varying the tune of said circuit at signal frequency comprising a pair of thermionic tubes having their input electrodes coupled to a source of modulating potentials, a reactance coupling the anode of one of said tubes to the reactance in said tuned circuit and a second reactance coupling the anode of the other of said tubes to the reactance in said tuned circuit.

6. A device for relaying carrier frequency oscillations and for modulating the phase of the oscillations relayed at, signal frequency comprising, a thermionic tube having its input electrodes energized at the frequency of said carrier frequency oscillations, a tuned circuit connected between the anode and cathode of said tube, a source of modulating potentials, a pair of thermionic tubes having their control grids coupled to said source of modulating potentials, an inductive reactance coupling the anode of one of said tubes to said tuned circuit and a capacitive reactance coupling the anode of the other of said tubes to a point on said tuned circuit.

'7. A device for relaying carrier frequency oscillations and for modulating the phase of the oscillations relayed at signal frequency comprising, a thermionic tube having its input electrodes energized at the frequency of said carrier frequency oscillations, a circuit including an inductance and condenser in parallel connected between the anode and cathode of said tube, a connection between the cathode of said tube and ground, a source of modulating potentials, a pair of thermionic tubes having their control grids coupled in phase opposition to said source of modulating potentials and their cathodes grounded, an inductive reactance coupling the anode of one of said tubes to the inductance in said parallel circuit and a capacitive reactance coupling the anode of the other of said tubes to a point on said parallel circuit.

8. A phase modulation system comprising a generator of carrier frequency oscillations, a tube having its input electrodes connected to said generator, a circuit having inductance and capacitance connected between the output electrodes of said tube, a load circuit coupled to said circuit having inductance and capacitance, an electron discharge device having its input electrodes coupled to a source of modulation potentials, and means coupling the output electrodes of said device to said circuit having inductance and capacitance whereby said modulation potentials cause changes in the effective reactance of said circuit having inductance and capacitance and phase modulation of the energy fed from said generator into said load circuit.

9. A system as recited in claim 8 wherein means is provided between said load circuit and said circuit having inductance and capacitance for rendering substantially constant in amplitude the phase modulated energy fed from said circuit having inductance and capacitance to said load circuit.

10. A system as recited in claim 8 in which said tube includes an auxiliary electrode and in which a circuit connects the auxiliary electrode to the cathode of the tube for reducing the effects of interelectrode capacity in the tube and in which means is provided between said load circuit and said circuit having inductance and capacitance for rendering substantially constant the phase modulated energy fed from said circuit having inductance and capacitance to said load circuit.

MURRAY G. CROSBY. 

